NEW YORK (Reuters) - A group of migrants seeking asylum in the United States sued the Trump administration on Thursday, claiming the government is unfairly keeping them in custody them while they pursue their cases in immigration court.

The class-action lawsuit on behalf of nine plaintiffs, brought by the American Civil Liberties Union and immigrant rights groups and filed in U.S. District Court in Washington, D.C., alleges five U.S. Immigration and Customs Enforcement (ICE) field offices have detained virtually all adults seeking asylum at a port of entry.

The Department of Homeland Security, which oversees ICE, said it does not comment on pending litigation and the Justice Department declined to comment.

President Donald Trump promised during his 2016 campaign to clamp down on illegal immigration and keep immigrants who contest deportation locked up during the process.

Under a 2009 ICE directive, border-crossers applying for asylum and having a "credible fear" of persecution or torture in their home countries can be released from detention on a case-by-case basis for humanitarian reasons.

The lawsuit said in 2013, 92 percent of asylum seekers found to have credible fear were promptly released from custody in the five field offices. Today, the lawsuit says, few are released.

U.S. Attorney General Jeff Sessions has said the asylum process has been "subject to rampant abuse and fraud" and that releasing immigrants as they pursue asylum claims created "incentives for illegal aliens to come here and claim a fear of return."

The practice of releasing asylum seekers is "still in place on paper" but "is effectively a dead letter" in practice, ACLU attorney Michael Tan said in an interview.

A recent decision by the U.S. Supreme Court found asylum seekers who crossed the border illegally do not have a right to a bond hearing in immigration court, making the need for ICE's policy of humanitarian parole all the more urgent, Tan said.

The lawsuit alleges that ICE offices in Detroit, Los Angeles, Philadelphia, El Paso, Texas, and Newark, New Jersey, are violating the U.S. Constitution and the agency's own policy guidelines by refusing the release most immigrants. The five offices oversee the detention of about one-quarter of the agency's average daily population of detainees.

The lead plaintiff is Ansly Damus, 41, an ethics teacher who fled political persecution in Haiti. He was twice granted asylum by a judge, according to the suit, but has remained locked up in Ohio for more than 16 months while the government appeals his case.